Hydraulic valve body issue I cant figure why, Nissan n350-2

[eddie57]

One day my nissan n350-2 mini excavator lost maximum power in the boom retract (digging in towards you) function. It had a leak in the cylinder, so I fixed it, no difference. I noticed the pressure on the retract side of the cylinder on a gauge was low, but it had a slight pressure all the time when running. I sent the valve body to a shop to be checked and rebuilt, they did it years ago and it worked after that. No difference ($1400 bill). I replaced all the filters while it was out. no diff. I checked the pilot pressure and all the lines and the supply safety valve were all in spec at 500 psi. I swapped the pressure relief valves around and around to make sure none of them were leaking, no difference. I pulled the valve out bc I thought it might be sticking - it was OOR .003" inch on the ends, I set up a grinding rig in my lathe and got it toal end to end within .001 - polished it and put it back in, no difference. I noticed the two pilot actuators that receive the 500 psi from the sticks were slightly different heights when sitting so I mic them and the pistons seemed to be slightly further out by .050" - I more carefully checked the springs in them and there were two different springs _ i thought I had it! I replaced them with two new springs that matched and made the sure the lengths were identical, no diff. I check the spring pressure of the actuator picton assembly figuring maybe the one side was pushing slightly and opening part of the port - no difference. I check the hydraulic valve and shuttle it though the bore all the way one side to the other no difference. I check the pressure on the one side the line to extend maxes at 3200 psi that is spec, the side with the issue where flow is constant but low pressure goes up to about 1200 psi max, but I tried swapping pressure reliefs again - no diff. all other lines are in spec psi. I look in the valve body there are no caps underneath on the under side, there are no other devices in this valve section, all the other functions on this side work. I tried removing the valve and turned it around and it made a very slight increase in power, but its still not like it was. I just cant see any other place the pressure could be going from that one circuit - the manual does not give me much more info, I contacted Nishiki (if I have that right) who made that valve and they said they used it a few machines back then but they dont have any blueprints left - all destroyed over aged. (1991 machine). the Japanese were friendly to me but said they got nothing. I took the valve body back to the hyd shop and they said its perfect and nothing is wrong, they said the pump is bad, but the pressures are all good everywhere else - so that seemed implausible. Here is some pictures of the valve body - the white pant is the one with the issue.

 

[eddie57]

more info - I swapped lines and the problem moved with the lines. But when I swapped pilot lines the problem did not move or change.

 

[eddie57]

If anyone knows this machine or its related machines or has Ideas I am stumped and for like a year now I have been trying things with no luck. I even tried bypassing the crossover lines in the valvebody and t'd a line from the pump supply directly to that side - no difference.
Somehow the fluid must be going somewhere from that side - any ideas? Any one have a internal valve block diagram? Experience? thank so much for your ideas in advance - your professionalism is far beyond mine here.

 

[eddie57]

test of springs in actuators and diagram of valve (sorry, im no artist). This shows the two different springs that were installed in the valve. Slightly different lengths but different rates. Now it has identical lenght and rate springs but that made no difference in performance. I was figuring the different rates and lengths were pushing the valve slightly to one side but it did not result.

 

[eddie57]

Would anyone know if there is a gasket between ports of each valve section of the assembled body where if the gasket blew out it would allow a constant flow and also a reduced full pressure? i HAVE THE nISSAN n350 SERVICE MANUAL and i was able to find a diagram of the valves but they show only one oring ealing each section together and no other bypass relief. If this oring were bad it would leak externally. It does not seem like the valve iteself because when reversed in the bore it does the same.
Any ideas ?

 

[ebaikie]

If the problem moved with the lines and it's always on the boom-down line, then you've isolated it to that circuit, not the control valve. Is there a "anti-drift" counterbalance load-holding valve on the boom cylinder? If so the seal could be gone between the pilot section (it would pilot from boom-down) and the boom-up section of the counterbalance valve cartridge. If there is no load holding valve on this machine then the problem must be with the piston seals in the hydraulic cylinder. I have seen piston seals fail to seal in one direction only.

 

[eddie57]

In the diagram, manual and physically looking at the machine there are no other devices between the control valve and the cylinder, each line is a home run.

I will look into the piston seals and pull it apart to check, the seals were replaced about 2 years ago so I will check them visually.

The thing I dont understand is why the boom down line when not active has flow of about 1 or 2 gpm, but no pressure on a gauge, while the boom up line has no flow when not active and no pressure on gauge. The leak in the seals would explain the low pressure on boom down pilot command, but after checking the control valve, reversing it in its bore, nothing changes so I wonder where the flow from the pump is coming from, unless the valve block itself had a defect? Or is it possible the cylinder seal leak could allow the valve to shuttle over being overcome by some sort of pressure pressure against it? It is not enough to move the boom at all, but when I disco the line it really flows. The pilot side has zero pressure and when disc zero flow and full pressure and flow vol when activated.

Thanks for your advice - I will follow up with you after I pull the cylinder apart

 

[ebaikie]

Before you pull the cylinder apart take both lines off the cylinder and plug them then test pressure again. If you have full 3200 psi in both directions with the lines plugged then the problem is definitely in the cylinder.

 

[eddie57]

Here it is, in the area you suspected. Some sort of damage. I dont know what the teflon seal on top of the piston is for. when I look up into the cyl I see where it fits into the top. The manual does not detail this as a separate part. Does this simply cut off flow flow when the cyl is fully retracted? In any event the cylinder has wear inside near the top and while I can build up and cut the piston on the lathe to remake it, the barrel will probably need to be cut off, welded up, re bored and honed at the top, rewelded back on, I dont have the right equipment to do that perfectly. I am looking for a used Nissan n-350-2 stick cylinder. If anyone has one (also Hynix made the same machine, mostly sold in EU) let me know, Used, new, rebuilt...
Thank you ebaikie for your accurate advice - you nailed the location of the problem. Strong work my friend.

 

[eddie57]

Now I am looking for a cylinder barrel repair shop. The damage is all the way up in the top of the cylinder. The piston I can easily weld up and re machine to work, but the damage in the cylinder I can not reach. The barrel will need to be cut off, braised or welded, machined and honed then put back together. If anyone in the north east knows a hydraulic shop who does this let me know, I am in the Newburgh NY area where 84 and I87 intersect.

 

[HarleyHappy]

Most any decent Hydraulic shop will be able to do this work. Finding one in a boneyard will be tough.
Might be able to retrofit another cylinder the same size.
I had a barrel made once and with today’s shop rate, could never afford it again.